Membrane filters of this general type that are known in the art are configured for filtering waste water that is heavily loaded with solids like they can be found for example in biologic waste water treatment plant in membrane bioreactors (MBR). The membrane filter can either be submerged in the tanks of the waste water treatment plant or can be provided with inlet and outlet conduits and can be set up on dry land integrated in a pipe. The driving force for the filtration is implements in most cases through a vacuum applied on the permeate side but can also be implemented for the dry set up variant by a low feed side (raw water side) positive pressure.
The membranes that are fixated in the membrane carrier can be in particular hollow fiber membranes which have a diameter of less than 5 mm but they can also be flat membranes. Thus a portion of the membrane filter is designated as the membrane carrier in which portion the membranes are fixated. Hollow fiber membranes are typically attached at least on a bottom in a membrane carrier, typically additionally also on top in a second membrane carrier. Also membrane filters with flat membranes include membrane carriers at which the flat membranes are attached. The flat membranes themselves have a permeability of microfiltration membranes or ultra-filtration membrane. Using membranes for reverse osmosis or nano filtration is possible. Typically hollow fiber membranes in a diameter range of 0.5-3 mm are being used.
In order to prevent a blocking of the membrane filters by filtered materials the membrane filter is flushed continuously or in periodic internals. Typically used physical flushing methods for the membrane filter use a permeate side back flushing of the membranes with liquid or gas combined with a gas bubble flushing on an outside of the membranes. Rising gas bubbles typically also generate an upward flow of the liquid to be filtered which is designated as mammoth pumping effect. A shear force of the 2 face flow including gas and liquid generates a high level of turbulence which removes coatings from the membranes and flushes them out. In membrane bioreactors air is typically used as a gas.
A membrane filter of this type is known from JP 10 06 63834. Thus plural membrane carriers with membranes attached therein which are not specified in more detail are arranged above a gas distribution system which includes plural downward open and upward closed tubs which have walls with downward open vertical slots for distributing the gas into the liquid.
In the known membrane filter the tub has the shape of a downward open cuboid or half cylinder with slots that are laterally arranged in the wall, extend vertically and are open in a downward direction. The known membrane filter has plural gas inlets into the gas distribution system which respectively connect to an interior of the tubs from above through a sealing of the tubs.
Through the gas inlets a gas flows from above into the tubs and fills the tubs up to a portion of the height of the tubs with a gas cushion. Thus also the slots fill up to an identical level with gas since the slots are open in outward direction the gas flows out of the gas cushion through the gas filled portion of the slots laterally out of the tub and thus flows at several locations below the membranes into the liquid to be filtered. In order to be able to compensate variations in the gas volume the slots are typically sized for normal operations so that they are only partially filled with gas. The filling level of the tub with gas and thus also the filling level of the slots is a function of the gas volume flow that flows into the gas distribution system. For higher gas volume flows the gas backs up in the tub to a higher level and thus a larger portion of the slots is filled with gas, this means the flow through cross section for the gas increases and a higher volume of gas flows through the slots. Up to a complete back up of the tub the gas flows out of the slots evenly. Only when the gas volume becomes large enough so that the tub flows over the additional gas volume exits from the tub in an uncontrolled manner.
After the gas flows out of the slots the gas subsequently rises in the membrane filter and thus generates an upward movement of the liquid through the membrane filter according to the mammoth pumping principle. The high shear force effect of the 2 face flow including the rising liquid and the gas thus flushes the membranes, wherein coatings and deposits are removed and carried out of the filter.
During lateral flow through the slots the gas generates a liquid flow that is oriented parallel to the lateral gas flow at a face boundary below the gas cushion wherein the liquid flow impacts the portion of the wall between the slots that protrudes on a bottom out of the gas cushion. This flow typically flushes in hair or fibrous compounds in membrane bioreactors, in particular in applications for municipal waste water processing.
In the membrane filter described in JP 10 06 6834 sections of the wall between the slots protruding from a bottom of the gas cushion act like a comb or rake upon hair and fibrous compounds included in the liquid to be filtered wherein the hair and fibrous compounds easily lodge in the slots. When the hair is carrier for example by the flow with one end into one slot and with another end into an adjacent slot the hair is retained at flow leading edges of the wall between the slots which can lead to a blocking of the slots. Thus the gas volume flowing through these slots is obstructed up to a complete blockage. As a consequence insufficient gassing and flushing is provided for a membrane portions that are arranged there above which creates a risk of blocking these portions.